Mycoplasmal Pneumonia of Swine

Introduction

Respiratory diseases are one of the leading causes of health issues, poor productive performance and mortality in growing and finishing pigs. Pathogens of various types can cause respiratory diseases, including but not limited to bacteria, viruses, and parasites. Prevention, diagnosis, control and elimination of respiratory diseases are critical to provide animal welfare, and to allow the expression of the pig’s full production potential. This factsheet summarizes current information of mycoplasmal pneumonia in swine, an important contributor to respiratory disease in pigs.

Objectives

Overview of mycoplasmal pneumonia infections in pigs and its importance for swine production
Confirmation of infection based on clinical presentation and laboratory-based tests
Current approaches for disease control

What causes Mycoplasmal pneumonia?

The causative agent of mycoplasmal pneumonia in swine is Mycoplasma hyopneumoniae (abbreviated M. hyopneumoniae), which was first identified over 50 years ago. M. hyopneumoniae is a very small bacterium that only affects pigs. Humans do not get sick from pigs infected with M. hyopneumoniae nor from being exposed in facilities in which infected pigs have been housed.

Pigs infected with M. hyopneumoniae develop a compromised integrity of the mucociliary apparatus, leading to inefficient removal of inhaled particles from the lungs. Thus, pigs with Mycoplasmal pneumonia are more prone to secondary infection of other respiratory pathogens. Mycoplasma infection alone is not very common. Other bacteria or viruses can also affect pigs at the same time, contributing to more complicated polymicrobial diseases. Generally, M. hyopneumoniae infections can be observed in combination with one or more bacterial agents such as Pasteurella multocida, Actinobacillus pleuropneumoniae, Streptococcus suis, Mycoplasma hyorhinis or Glaeserella parasuis. Likewise, viral agents like porcine reproductive and respiratory syndrome virus (PRRSV), influenza A virus (IAV) or porcine circovirus type 2 (PCV2), serve as frequent co-infections together with M. hyopneumoniae, leading to a condition known as Porcine Respiratory Disease Complex (PRDC). Whether alone or in co-infections, M. hyopneumoniae is a pathogen playing a crucial role in pig respiratory disease.

Is Mycoplasmal pneumonia important in swine production?

Mycoplasmal pneumonia itself may not be the most worrisome malady of swine. However, due to its central role in disease causation and the fact that co-infections are so common, M. hyopneumoniae infections have been estimated to significantly increase the cost of pig production. In the most recent decade, data from various production systems looking into different outcomes has estimated that every pig in a herd infected with M. hyopneumoniae costs an average $5 more to reach market age than the Mycoplasma negative counterpart.

How do pigs get infected?

Pigs are essentially born free of M. hyopneumoniae infection. However, early exposure to the bacterium occurs and seems to be key in the perpetuation of infections in swine herds. Pig-to-pig transmission of M. hyopneumoniae through respiratory secretions appears to be very effective. Sows can shed the pathogen and expose the offspring early in life. Colonized piglets may not develop disease immediately, but in turn may harbor the bacterium in their respiratory tract, which will grow at a slow rate, with pigs shedding the agent during the growing phase and exhibiting clinical signs associated with the infection mainly in the finishing period. The level of M. hyopneumoniae colonization in piglets at weaning has been correlated to disease severity at market. In other words, groups of weaning pigs with low colonization levels may not develop disease that can be evidenced in the growing period, but piglets with a high colonization level at weaning age may develop disease early in the production cycle. Thus, the shedding from sows to piglets is an important measure for disease transmission, and is of paramount importance in the epidemiology of M. hyopneumoniae.

Transmission of M. hyopneumoniae among penmates is considered the main way of infection in large pig groups. The fact that pigs of all ages appear to be susceptible to infection is of special importance. Being an agent that enters the host through the respiratory tract, M. hyopneumoniae infections can occur via aerosol. Short and long distance aerosols loaded with M. hyopneumoniae have been shown to induce disease in naïve pigs. Nevertheless, the efficacy of transmission of M. hyopneumoniae seems to be lower than that of other respiratory agents, such as PRRSV.

Vectors and fomites do not seem to play a role in the transmission of M. hyopneumoniae. However, recent information suggests that the bacterium can be detected by PCR (polymerase chain reaction) and/or remain viable in the environment for relatively long periods of time (at least detected by PCR, which means it may or may not be viable but can be detected), depending on the surface material. Previously, it was thought that M. hyopneumoniae viability outside of the host was very limited.

What is the clinical presentation?

Infections caused by M. hyopneumoniae are chronic in nature. Studies looking into the duration of disease have documented pathogen detection and transmission surpassing seven months, which in essence goes beyond the entire production cycle of a market pig. When infected with M. hyopneumoniae, pigs may develop a dry cough, which is not unique to the disease. However, cough is not present at all times during infection. The M. hyopneumoniae associated cough can be actually observed approximately one month after initial exposure and lasts an average of 60 days. Pigs can remain infected and infectious to others, even after the cough has ceased.

Pigs infected with M. hyopneumoniae will show affected production parameters. Mainly, the number of days to reach market weight will be lengthened in infected pigs.

**Confirming M. hyopneumoniae detection or infection**

The diagnostic approach to M. hyopneumoniae infections will vary depending on the type of production unit and stage, disease status, and the final objective of the study. As a high proportion of herds are considered to be positive for M. hyopneumoniae, detecting the pathogen in the group of pigs may not be the main purpose of most diagnostic explorations. Therefore, aiming at detecting M. hyopneumoniae in a herd that has been negative to the agent will imply a much different approach than that taken in a previously positive grow-finish population.

Bacterial culture and isolation of this bacterium can be extremely difficult. The pathogen grows slowly, even in laboratory settings in which rich broth media are employed, and other environmental and growth conditions are optimized. Based on the slow replication rate of M. hyopneumoniae, most other bacterial agents present in the respiratory tract can overgrow it, making it virtually impossible to recover in clinical specimens. The reported success rate for bacterial isolation of M. hyopneumoniae does not reach 10%.

Detection of circulating antibodies to M. hyopneumoniae via enzyme-linked immunosorbent assays (ELISA) have been widely used. ELISA tests aid in the identification of exposure to the pathogen, whether by infection or vaccination, which can complicate diagnostic interpretations as most commercial herds are vaccinated for this bacterium. Moreover, it is important to mention that circulating antibodies are not detected right after M. hyopneumoniae exposure or until the end of the infection. Thus, care should be taken when attempting to pinpoint infection or infectiousness based on ELISA results.

The observation of lung lesions suggestive of M. hyopneumoniae infection can be very useful in diagnostics. Although not pathognomonic, lung lesions develop several days to several weeks after exposure and can last a long time. Nevertheless, lesions usually heal prior to the end of the infection period, which can be misleading, especially when slaughterhouse lung evaluations are performed. Gross and microscopic assessment of lung lesions are well established, and scoring systems are commonly used. The main detractor of the use of lung lesions for diagnostic of M. hyopneumoniae is the fact that it is performed post-mortem. Imaging techniques for the evaluation of lung lesions in vivo have been assessed and show promising applications but are not widely available at this time.

Nowadays, PCR testing constitutes the most used approach for detection of M. hyopneumoniae. The high sensitivity and specificity of PCR makes it an ideal choice for most diagnostic investigations. Over the years, numerous specimen types and sample collection methods have been tested for M. hyopneumoniae detection, in combination with PCR. The sensitivity of detection of M. hyopneumoniae in clinical samples appears to be highly associated with the location of the agent in the respiratory tract of the pig. Thus, samples collected in the upper respiratory tract, or the oral cavity, such as nasal swabs and oral fluids, do not seem to provide good detection of the agent, even in individuals or groups of experimentally infected pigs. Conversely, specimens obtained from the lower respiratory tract, like tracheal and bronchial secretions, provide high sensitivity. Of course, bronchial swabs are collected post-mortem, thus their application is limited to certain investigations. However, tracheal sampling has become a routine technique in swine herds and its application has evolved to make it more practical, although it is still far from easy and requires specific training. It cannot be forgotten that the advantage of the PCR can become a liability at times. The high sensitivity of the assay can puzzle swine professionals, as M. hyopneumoniae detection has been obtained in environmental samples or specimens from body sites other than the respiratory tract. Thus, the bottom line is to exercise extreme care to avoid pre-clinical cross-contamination when collecting samples to be processed by PCR.

**How to control M. hyopneumoniae infections**

The pillars of M. hyopneumoniae control are optimization of management practices, vaccination and antibiotic treatment. Generally, a combination of the three measures is employed in the field in order to obtain satisfactory results.

Management practices

Management practices should be one of the first aspects to review and improve when dealing with respiratory diseases in pigs, especially the illness caused by M. hyopneumoniae. Proper stock density, good ventilation, strategic pig flow and sound biosecurity are necessary for M. hyopneumoniae control and their application has shown to decrease the detrimental effect that the infection causes. Nowadays, a widely used management practice for M. hyopneumoniae disease control is exposure of naïve gilts. Briefly, a controlled purposeful exposure to the pathogen is performed in naïve gilts prior to entering an endemically infected sow farm, allowing ample time for them to cease shedding and transmission by the time they farrow their first litter , which has been shown to last approximately eight months. At the end of the exposure period, gilts are transferred to the sow farm under the premise that the previous infection and recovery provides protection from re-infection and they do not pose a risk for pathogen transmission and herd destabilization.

Vaccination

Vaccination against M. hyopneumoniae, with the use of commercial bacterins has proven to decrease issues associated with the infection and to improve production parameters. Vaccination has shown to lower the bacterial load at the lung level, with a subsequent decrease in clinical signs and lung lesions, and tends to decrease pathogen shedding that can lead to pig-to-pig transmission. Most commercial vaccines are adjuvanted killed preparations that are administered either intramuscularly or intradermally. Piglets are usually vaccinated at weaning, while gilts are vaccinated prior to entering the sow farm. Single or double dose vaccination protocols are common.

Antibiotic treatment

Antibiotic treatment is an important tool to combat mycoplasma associated infections, although they cannot be cured. As M. hyopneumoniae is a bacterium, a large number of antibacterial compounds can be used for treatment of the disease it causes. However, it is important to remember that Mycoplasma species lack a cell wall, thus they are naturally resistant to antibiotics that target the cell wall, such as penicillins and other β-lactams. Several antibiotics can be used to treat M. hyopneumoniae infections, and only the herd’s attending veterinarian can prescribe an appropriate antibacterial drug. The decision on the antibacterial treatment is based on multiple factors that may include, but it is not limited to, route of administration, time after infection, previous response to treatment, antibacterial resistance information, withdrawal period, and cost.

Can the disease be eliminated?

Yes, M. hyopneumoniae infections can be eliminated from swine farms. In the past, elimination programs were thought to be reserved for multiplication sites or very small farms. Today, more and more commercial herds embark on a mycoplasma elimination journey thanks to the design and development of programs that are adapted to large swine farms. Besides depopulation and repopulation of the farm, two main programs used in North America for M. hyopneumoniae elimination are Herd closure and medication, and Whole herd medication in breed-to-wean farms. The success rate of M. hyopneumoniae elimination varies depending on the method that is used, the program implementation, the particularities of the production unit, and potentially on the virulence of the bacterial strain. This can be performed with relatively short period of time for payback or return on investment.

Summary

Mycoplasmal pneumonia is highly prevalent and continues to be a concern for swine producers and practitioners. The disease affects the respiratory tract of pigs and mainly decreases the production performance of finishers. Moreover, mycoplasmal pneumonia opens the door to a variety of pathogens, which can exacerbate respiratory problems and contributes to the Porcine Respiratory Disease Complex. However, because of its slow growth, infections start early in the life of pigs and clinical signs are observed later on in the growing phase. Knowing how to recognize and diagnose infections is critical for the application of proper control measures. Management optimization, vaccination and medication can be used to combat the effects of mycoplasmal pneumonia, but the ultimate control is disease eradication. Nowadays, efforts are underway in numerous US swine herds and production systems to eradicate mycoplasmal pneumonia. Several disease eradication strategies can be used for that purpose.

References and Citations

Pieters MG, Maes D (2019). Mycoplasmosis. In: Diseases of Swine (pp. 863–883). Eleventh edition. Blackwell Publishing Group. Hoboken, NJ, USA: John Wiley & Sons, Inc. https://doi.org/10.1002/9781119350927.ch56
Maes. D., Sibila, M., Pieters, M (editors). Mycoplasmas in Swine (344 pp). Edition 1. Acco. ISBN 9789463797962